Coating to cool a surface by passive radiative cooling

ABSTRACT

Disclosed herein in is a radiative cooling formulation comprising a first component with &gt;55% reflectance in a wavelengths range of 0.3 to 2.5 microns, a second component with &gt;0.85 peak thermal emissivity in a window range of 4 to 35 microns, and a third component to mechanically bind together a mixture of the first and second components.

RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 15/444,029, titled COATING TO COOL A SURFACE BY PASSIVE RADIATIVECOOLING, filed Feb. 27, 2017, the entire contents of which are herebyincorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with Government support under Contract No.DE-AR0000583ARPAE-ARID-MetaCool awarded by the Advanced ResearchProjects Agency-Energy. The Government has certain rights in thisinvention.

TECHNICAL FIELD

The presently disclosed implementations are directed to a coating, and,more particularly, to a coating to cool a surface by passive radiativecooling.

BACKGROUND

Cooling can be achieved by active cooling or passive cooling. Activecooling involves the consumption of energy to cool an object (e.g.,paying for external energy), whereas passive cooling requires no energyfrom an external source to cool an object (e.g., natural, no-cost energytransfer). Radiative cooling is the process by which an object losesheat by thermal radiation (e.g., electromagnetic radiation generated bythermal motion of charged particles in matter). Passive radiativecooling refers to losing heat by thermal radiation to an externalthermal sink, without the consumption of energy. A subset of passivecooling systems operate even when the objects to be cooled are exposedto sunlight. Such daylight passive radiative cooling materials includefor example multilayer inorganic films, coating formulations includingglass microspheres, and multilayer polymer films with a silverreflector.

SUMMARY

The following presents a simplified summary of various aspects of thepresent disclosure in order to provide a basic understanding of suchaspects. This summary is not an extensive overview of the disclosure. Itis intended to neither identify key or critical elements of thedisclosure, nor delineate any scope of the particular implementations ofthe disclosure or any scope of the claims. Its sole purpose is topresent some concepts of the disclosure in a simplified form as aprelude to the more detailed description that is presented later.

According to some aspects of the present disclosure, there is provided aradiative cooling formulation including a binder comprising a pluralityof polymers including a first polymer and a second polymer that arepractically water insoluble and are substantially non-absorbing to lighthaving wavelengths in a solar spectrum. The first polymer has a firstemissivity peak value greater than 0.85 at a first wavelength between 4and 35 micrometers (μm) and the second polymer has a second emissivitypeak value greater than 0.85 at wavelengths between 4 and 35 μm. Thefirst emissivity peak value and the second emissivity peak value aresubstantially non-overlapping. A net emissivity of the first polymer andthe second polymer is greater than at least one of first emissivity ofthe first polymer or second emissivity of the second polymer. Theradiative cooling formulation further includes a solar reflectormaterial embedded in the binder.

In certain implementations, each of the plurality of polymers has acorresponding, emissivity peak value greater than 0.85 at wavelengthsbetween 4 and 35 μm and each of the corresponding emissivity peak valuesare substantially non-overlapping.

In certain implementations, the first wavelength and second wavelengthare between 8 and 13 μm.

In certain implementations, the solar reflector material reflects solarradiation at wavelengths from 0.3 to 2.5 μm and has an average solarreflectance greater than 0.95.

In certain implementations, the solar reflector material comprisesparticles of barium sulfate (BaSO₄) and at least half of the particlesof BaSO₄ are smaller than 2 μm.

In certain implementations, the radiative cooling formulation furthercomprises titanium dioxide (TiO₂) embedded in the binder.

In certain implementations, each of the plurality of polymers isselected from the group consisting of ethyl cellulose, poly ethylmethacrylate (PEMA), poly methyl methacrylate (PMMA), polyvinyl butyral(PVB), cellulose acetate, polyethylene, polypropylene, polyethyleneterephthalate (PET), polyethylene naphthalate (PEN), polyesters, andpolycarbonates, wherein the first polymer is different from the secondpolymer.

In certain implementations, the binder is a polymer emulsion comprisingat least one of a first set of particles comprising the first polymer ora second set of particles comprising the second polymer.

According to other aspects of the present disclosure, there is providedan apparatus including a substrate and a coating on the substrate. Thecoating includes a binder including a plurality of polymers including afirst polymer and a second polymer that are practically water insolubleand are substantially non-absorbing to light having wavelengths in asolar spectrum. The first polymer has a first emissivity peak valuegreater than 0.85 at first wavelength between 4 and 35 μm and the secondpolymer has a second emissivity peak value greater than 0.85 at a secondwavelength between 4 and 35 μm, the first emissivity peak value and thesecond emissivity peak value are substantially non-overlapping, and anet emissivity of the first polymer and the second polymer is greaterthan at least one of first emissivity of the first polymer or secondemissivity of the second polymer. The coating further includes a solarreflector material embedded in the binder.

In certain implementations, each of the plurality of polymers has acorresponding emissivity peak value greater than 0.85 at wavelengthsbetween 4 and 35 μm and each of the corresponding emissivity peak valuesare substantially non-overlapping.

In certain implementations, the first wavelength and second wavelengthare between 8 and 13 μm.

In certain implementations, thickness of the coating is at least 70 μm.

In certain implementations, the apparatus further includes a layer onthe coating, the layer including one or more of polytetrafluoroethylene(PTFE), perfluoroalkoxy polymer (PFA), fluorinated ethylene-propylene(FEP), ethylene tetrafluoroethylene (ETFE), ortetrafluoroethylene/hexafluoropropylene/vinylidene fluoride copolymer(THV).

In certain implementations, the apparatus further includes a layerincluding a hydrophobic material, the layer being on the coating. Thehydrophobic material is substantially non-absorbing of wavelengths from0.3 to 2.5 μm and the hydrophobic material includes at least one offluorinated silica nanospheres or nano-etched silica.

In certain implementations, the apparatus further includes a layerincluding TiO₂, the layer being on the coating.

In certain implementations, the coating provides passive radiativecooling greater than zero Watts per square meter (W/m²) without input ofwater or electricity.

In certain implementations, the coating provides passive radiativecooling of at least 5 degrees Celsius below ambient temperature whensolar illumination is present and without input of water or electricity.

In certain implementations, the substrate comprises at least one ofaluminum, steel, galvanized steel, carbon fiber resin, a tent, aflexible tarp, a roof structure, or a surface of an automobile.

According to other aspects of the present disclosure, there is provideda method comprising applying a coating of a radiative coolingformulation to an object. The radiative cooling formulation includes abinder including a plurality of polymers including a first polymer and asecond polymer that are practically water insoluble and aresubstantially non-absorbing to light having wavelengths in a solarspectrum. The first polymer has a first emissivity peak value greaterthan 0.85 at a first wavelength between 4 and 35 μm and the secondpolymer has a second emissivity peak value greater than 0.85 atwavelengths between 4 and 35 μm. The first emissivity peak value and thesecond emissivity peak value are substantially non-overlapping. A netemissivity of the first polymer and the second polymer is greater thanat least one of first emissivity of the first polymer or secondemissivity of the second polymer. The radiative cooling formulationfurther includes a solar reflector material embedded in the binder.

In certain implementations, each of the plurality of polymers has acorresponding emissivity peak value greater than 0.85 at wavelengthsbetween 4 and 35 μm and each of the corresponding emissivity peak valuesare substantially non-overlapping.

In certain implementations, the first wavelength and second wavelengthare between 8 and 13 μm.

In certain implementations, the method further includes applying a layeron the coating, the layer including one or more of PTFE, PFA, FEP, ETFE,or THV.

In certain implementations, the method further includes applying a layeron the coating, the layer including a hydrophobic material, where thehydrophobic material is substantially non-absorbing of wavelengths from0.3 to 2.5 μm.

In certain implementations, the method further includes applying a layeron the coating, the layer including TiO₂.

In certain implementations, the object includes at least one ofaluminum, steel, galvanized steel, carbon fiber resin, a tent, aflexible tarp, a roof structure, or a surface of an automobile.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure described herein is illustrated by way of exampleand not by way of limitation in the accompanying figures. For simplicityand clarity of illustration, features illustrated in the figures are notnecessarily drawn to scale. For example, the dimensions of some featuresmay be exaggerated relative to other features for clarity. Further,where considered appropriate, reference labels have been repeated amongthe figures to indicate corresponding or analogous elements.

FIG. 1A illustrates an apparatus including a coating of a radiativecooling formulation on a substrate, in accordance with implementationsof the present disclosure.

FIG. 1B illustrates an apparatus including the coating on the substrateand a first layer on the coating, in accordance with implementations ofthe present disclosure.

FIG. 1C illustrates an apparatus including the coating on the substrateand the first layer and a second layer on the coating, in accordancewith implementations of the present disclosure.

FIG. 1D illustrates an apparatus including the coating on the substrateand the first layer, the second layer, and the third layer on thecoating, in accordance with implementations of the present disclosure.

FIG. 2A is a graph illustrating power absorbed by an apparatus with acoating of a radiative cooling formulation, power radiated by theapparatus, solar power incident on the apparatus, and net cooling powerof the apparatus at different surface temperatures, in accordance withimplementations of the present disclosure.

FIG. 2B is a graph illustrating ambient temperature and temperature of asubstrate coated with a radiative cooling formulation at different timesof day, in accordance with implementations of the present disclosure.

FIG. 3A is a graph illustrating linear regression of change oftemperature of a cool roof paint and linear regression of change oftemperature of a coating of a radiative cooling formulation at differentvalues of solar illumination, in accordance with implementations of thepresent disclosure.

FIG. 3B is a graph illustrating linear regression of change oftemperature of a cool roof paint and linear regression of change oftemperature of a coating of a radiative cooling formulation at differentvalues of ambient temperature, in accordance with implementations of thepresent disclosure.

FIG. 3C is a graph illustrating linear regression of change oftemperature of a cool roof paint and linear regression of change oftemperature of a coating of a radiative cooling formulation at differentvalues of relative humidity, in accordance with implementations of thepresent disclosure.

FIG. 3D is a graph illustrating linear regression of change oftemperature of a cool roof paint and linear regression of change oftemperature of a coating of a radiative cooling formulation at differentvalues of wind speed, in accordance with implementations of the presentdisclosure.

FIG. 3E is a graph illustrating linear regression of change oftemperature of a cool roof paint and linear regression of change oftemperature of a coating of a radiative cooling formulation at differentvalues of cloud cover, in accordance with implementations of the presentdisclosure.

FIG. 4A is a graph illustrating emissivity at different wavelengths forpoly ethyl methacrylate (PEMA) and polyvinyl butyral (PVB), inaccordance with implementations of the present disclosure.

FIG. 4B is a graph illustrating temperature of a coating of a radiativecooling formulation including one polymer, temperature of a coating of aradiative cooling formulation including two polymers, and ambienttemperature at different times of day, in accordance withimplementations of the present disclosure.

FIG. 4C is a graph illustrating solar reflectance of coatings of aradiative cooling formulation of different thicknesses, in accordancewith implementations of the present disclosure.

FIG. 4D is a graph illustrating solar reflectance of coatings of aradiative cooling formulation including solar reflector material ofdifferent particle sizes, in accordance with implementations of thepresent disclosure.

FIG. 5A is a flow diagram illustrating a method of applying a coating ofa radiative cooling formulation to an object, in accordance withimplementations of the present disclosure.

FIG. 5B is a flow diagram illustrating a method of applying a coating ofa radiative cooling formulation to an object and one or more layers onthe coating, in accordance with implementations of the presentdisclosure.

DETAILED DESCRIPTION

Described herein are implementations of a radiative cooling formulation(e.g., a passive radiative cooling formulation, a radiative coolingpaint, a radiative cooling coating, etc.), an apparatus including asubstrate coated with a radiative cooling formulation, and methods ofapplying a radiative cooling formulation to an object. In certainimplementations, the radiative cooling formulation can be used toachieve cooling (e.g., from 0 (equilibrium, e.g., at 8° C. belowambient) or greater than 0 to about 400 W/m², depending on air andsubstrate temperature) without any input of water or electricity. Theradiative cooling formulation can achieve cooling of up to about 10degrees Celsius (°C.) below ambient temperature during daytime. Incertain implementations, the radiative cooling formulation is low cost(e.g., does not rely on lithography or multilayer dielectric thin films)and is scalable (e.g., a 100-500 μm-thick coating can be applied bybrushing, spraying, rolling, dipping, doctor blading a paint, or otherpainting technologies).

FIGS. 1A-D illustrate an apparatus 100 including a coating 120 of aradiative cooling formulation on a substrate 110, in accordance withimplementations of the present disclosure.

The substrate 110 may include one or more of metal, ceramic, plastic,composite, stationary parts, moving parts, etc. For example, thesubstrate 110 may include aluminum, steel, galvanized steel, carbonfiber resin, a roof tent, a flexible tarp, an upper layer of a roofstructure, an outer surface of an automobile, an interior surface (e.g.,cabin surface) of an automobile, etc. The substrate 110 may be used inbuilding applications, power plant cooling, cooling water in pipes(dissipate heat from the water to the sky 108 so that T_(out) of thewater is less than T_(in) of the water), tents, umbrellas, blankets,etc.

In sonic implementations, the substrate 110 is a roof and the coating120 is applied to the roof to cool the roof (e.g., a “cool roof”technology, the coating 120 is a cool roof coating, a “cool roof” is aroof that has been designed to reflect more sunlight and absorb lessheat than a standard roof). The coating 120 (e.g., cool roof coating)may have a pigment (e.g., a white pigment, a reflective pigment) thatreflects sunlight. The coating 120 may protect the substrate 110 (e.g.,roof surface) from ultraviolet (UV) light and chemical damage and mayoffer water protection and restorative features.

The coating 120 is a radiative cooling formulation that is applied tothe substrate 110 in one single layer. The radiative cooling formulationmay be applied to the substrate 110 in the form of paint (e.g., sprayed,brushed, rolled, dipped, doctor bladed, etc.) to form the coating 120.In some implementations, the coating 120 is ultra-white. The radiativecooling formulation provides solar reflectance and infrared emissivityin one single layer that can be sprayed on as coating 120. The radiativecooling formulation includes a binder and a solar reflector material(e.g., solar reflector particles) embedded in the binder (e.g., thebinder binds solar reflector material together).

The coating 120 can either be a broadband thermal emitter (i.e. providesthermal emission at all wavelengths>4 μm) or a selective emitter (e.g.,emits thermal radiation in the atmospheric transparency window (e.g.,8-13 μm)). In both cases, the coating 120 cools down by emitting morethermal energy (thermal emission 106, power radiated 206 of FIG. 2A) toits surroundings (e.g., sky (cold sink) 108) than the thermal energy(thermal emission 107, power absorbed 202 of FIG. 2A) that the coating102 absorbs from the sky 108 (e.g., the sky 108 is a cold sink). Abroadband thermal emitter may maximize the cooling power and may achievea temperature of about 10° C. below ambient temperature. A selectiveemitter may maximize the temperature difference relative to ambient airtemperature and may achieve a temperature of about 60° C. below ambient(e.g., temperature difference may depend on amount of energy loss due toconvection, conduction and radiation). Selective emission may not beneeded when the temperature of the coating 120 is more than 10° C. belowambient temperature and selective emission may be needed when thetemperature of the coating 120 is less than 10° C. below ambienttemperature.

The binder includes two or more polymers (e.g., a first polymer and asecond polymer). In certain implementations, the two or more polymersare practically water insoluble (e.g., once dried or cured arepractically not soluble in water, about greater than 10,000 mL water isneeded to dissolve 1 g of the two or more polymers that are practicallywater insoluble). In certain implementations, the two or more polymersare water insoluble.

The two or more polymers are substantially non-absorbing (e.g., solarabsorbance of the polymers is less than 0.7 when the coating 120 has athickness of at least 1 millimeter (mm)) to light having wavelengths ina solar spectrum (e.g., 200-4000 nanometers (nm), 400-700 nm, 450-650nm, etc.). In certain implementations, the two or more polymers' haveemissivity peak values greater than 0.85 at wavelengths between 4 and 35μm. In certain implementations, the two or more polymers have emissivitypeak values greater than 0.85 at wavelengths between 8 and 13 μm. Two ormore of the emissivity peak values are substantially non-overlapping(e.g., a first emissivity peak value of a first polymer is at a firstwavelength and a second emissivity peak value of a second polymer is ata second wavelength which is substantially different from the firstwavelength). A net emissivity of the two or more polymers is greaterthan emissivity of any one of the two or more polymers alone (e.g.,increases the aggegate emissivity in the range of 4-35 μm, a netemissivity of the first polymer and the second polymer at a thirdwavelength between 4 and 35 μm is greater than emissivity of either ofthe first polymer or the second polymer alone at the third wavelength).

In certain implementations, the two or more polymers are two or more ofethyl cellulose, poly ethyl methacrylate (PEMA), poly methylmethacrylate (PMMA), polyvinyl butyral (PVB), cellulose acetate,polyethylene, polypropylene, polyethylene terephthalate (PET),polyethylene naphthalate (PEN), polyesters, and polycarbonates, where afirst polymer of the two or more polymers is different from a secondpolymer of the two or more polymers. The two or more polymers may alsoincorporate ultraviolet absorbers additives that absorb UV radiation(e.g., wavelengths ≤400 nm). Ultraviolet absorbers suitable for thepresent invention include hydroxybenzophenone andhydroxyphenylbenzotriazole, titanium dioxide, benzotriazoles andhydroxyphenyltriazines.

In certain implementations, the binder in the coating 120 has a highspectral emittance (e.g., emissivity peaks) at wavelengths whereinfrared radiation occurs for a blackbody at a temperature near 300Kelvin (K) (e.g., wavelengths of 4 to 35 μm). In certainimplementations, the binder in the coating 120 has a high spectralemittance (e.g., emissivity peaks) at wavelengths for which theatmosphere is transparent (8 to 13 μm). This results in an imbalancebetween thermal radiation emitted by the sample, and that emitted by theatmosphere and absorbed by the binder in the coating 120. The imbalanceprovides a net cooling of the binder, coating 120, substrate 110, andapparatus 100.

The coating 120 reflects sun rays 102 (e.g., wavelengths in the solarspectrum, wavelengths of 0.3 to 2.5 μm) to produce reflected sun rays104. In certain implementations, the sun rays 102 are scattered by thetwo or more polymers of the binder. In certain implementations, the sunrays 102 are propagated through the two or more polymers and scatteredby the solar reflector material. In certain implementations, a firstportion of the sun rays 102 are scattered by the two or more polymersand a second portion of the sun rays 102 are propagated through the twoor more polymers and scattered by the solar reflector material, leadingto a solar reflectance of coating 120 that is higher than 0.95.

In some implementations, the solar reflector material reflectswavelengths in the solar spectrum (e.g., sun rays 102). The solarreflector material may reflect solar radiation (wavelengths of 0.3 to2.5 μm) with an average solar reflectance that is higher than 0.95. Theheat input due to absorbed solar radiation is less than the heat lossdue to emitted thermal radiation thus resulting in a net heat loss forthe coated object. For example, for a typical solar illumination of 900W/m², a solar reflectance of the coating 120 equal to 0.95 results inheating of the object at a power density of 45 W/m². The net coolingpower from thermal emission (thermal radiation power density emitted bythe apparatus 100 minus thermal radiation power density absorbed fromthe atmosphere at a surface temperature, and an ambient air temperatureof 30° C.) is about 100 W/m². In this case, the apparatus temperature isreduced due to the imbalance (e.g., net power into the object is 45W/m²−100 W/m²=−55 W/m²).

In certain implementations., the solar reflector material is a pigmentthat can be used in paint. In certain implementations, the solarreflector material includes particles of barium sulfate (BaSO₄, barite,blanc fixe). Using BaSO₄ in the coating 120 may provide a solarreflectance of the coating 120 of up to 97%. BaSO₄ has high spectralemissivity at 8.4, 8.9, and 9.3 μm, and so contributes to infraredradiation (IR) emission in the atmospheric transparency window. In oneimplementation, at least half of the particles of BaSO₄ are smaller than2 μm. In one implementation, at least half of the particles of BaSO₄ areno larger than 5 μm. In one implementation, at least half of theparticles of BaSO₄ are not smaller than 0.2 μm. In anotherimplementation, at least half of the particles of BaSO₄ are not smallerthan 0.1 μm. In one implementation, the particles of BaSO₄ have a meanparticle size (volume) distribution (D50(volume)) from about 0.1 toabout 5 μm. In one implementation, the particles of BaSO₄ have aD50(volume) from about 0.2 to about 2 μm. In one implementation, 0.0% ofthe particles of BaSO₄ fall below 0.1 μm. In one implementation, 0.0% ofthe particles of BaSO₄ are above 5 μm.

The solar reflector material particles may have a distribution of sizesand morphologies (e.g., related to the method of manufacture). In oneimplementation, the solar reflector material includes one or more ofspherical particles, flake particles, or elongated particles. In anotherimplementation, the solar reflector material comprises particles ofpolytetrafluoroethylene (PTFE). In another implementation, the solarreflector material comprises one or more of BaSO₄, PTFE, zinc oxide(ZnO), aluminum oxide (Al₂O₃, alumina), magnesium oxide (MgO, magnesia).

In certain implementations, the coating 120 of radiative coolingformulation further includes titanium dioxide (TiO₂) embedded in thebinder. In certain implementations, the TiO₂ is a layer (e.g., firstlayer 130, second layer 140, third layer 150) on the coating 120. TheTiO₂ may perform photocatalytic degradation of particles, gases, andpollutants that would otherwise result in increased solar absorbance anddecreased solar reflectance.

In some implementations, the radiative cooling formulation includes lessthan about 1% by volume of TiO₂, about 70 to 94% by volume of BaSO₄, andabout 6-30% by volume binders. In some implementations, the radiativecooling formulation includes a solvent that dissolves the two or morepolymers. The solvent would be selected not just based on its solventstrength, but also based on its safety, boiling and flash points, andprice. Examples include one or more of ethyl alcohol, butyl carbitol™,carbitol™, dimethylformamide, xylene, toluene, mineral spirits (e.g.mixture of aliphatic carbons), methylethyl ketone, methyl isobutylketone, butyl acetate, 1-methoxy-2-propylacetate, etc. In someimplementations, the binder is in the form of an emulsion and water maybe used as the liquid carrier.

In some implementations, the binder of the radiative cooling formulationincludes a polymer emulsion including at least one of a first set ofparticles including the first polymer or a second set of particlesincluding the second polymer. The binder may be a dispersion of one ormore polymers in a solvent (e.g., water) in which the one or morepolymers are not soluble or miscible. The binder (e.g., a polymermaterial) may he a water emulsion (e.g., incorporated as a wateremulsion instead of as dissolved in an organic solvent). A binder thatis a polymer emulsion may be used in environments requiring low volatileorganic content (VOC) coating materials. In sonic implementations, theradiative cooling formulation includes a dispersion of particles (e.g.,polymer binders. TiO₂, BaSO₄, dispersant, etc.) in water. In someimplementations, radiative cooling formulation includes about 6-30% byvolume of polymer binders in the form of nanoparticles, less than about1% by volume of TiO₂, about 70 to 94% by volume of BaSO₄, and less than5% by volume of dispersant. The emulsion may, include dispersantstabilized polymer particles having a particle size from about 10 nm to500 nm dispersed in water.

In some implementations, the polymer particle emulsion is prepared bydissolving the one or more polymers in an organic solvent by mixing theone or more polymers in the organic solvent with a mechanical stirrerand heating as necessary to obtain a homogeneous polymer solution. Anysolvent that dissolves the one or more polymers and that is either notmiscible or has relatively limited miscibility with water is suitablefor the purpose of fabrication of the emulsion. About 10% by volume ofNH₄OH is added to the polymer solution mixture drop-wise (e.g., drop bydrop, dripping the NH₄OH into the polymer solution) and the mixture isstirred for up to 15 minutes. De-ionized water (e.g., about 3-5 timesmore deionized water than the volume of organic solvents) is slowlymixed with a pipette or with a pump for at least 1-5 hours. The mixtureis poured into a glass pan, which is maintained in a fume hood overnightand is stirred by a magnetic stir-bar so that the solvent can evaporateoff. Alternatively, the solvent can be removed by a conventionaldistillation process.

In some implementations, the coating 120 of radiative coolingformulation further includes a hydrophobic material as a layer (e.g.,first layer 130, second layer 140, third layer 150) on the coating 120.Apparatus 100 may become dirty and accumulate dust, dirt, and debrisover time which degrades the solar reflectance (e.g., a cool rooftechnology may drop 5 to 23% in solar reflectance over 3 years).Apparatus 100 must be cleaned and kept free of dust, dirt, and debris tomaintain a high reflectance. The hydrophobic material (e.g., in or onthe coating 120) allows the apparatus 100 to be kept clean and free ofdust/debris to maintain a high reflectance over time (e.g., allowswashing of the apparatus 100, prevents liquid accumulation, repelswater, repels ice, etc.). The hydrophobic material is substantiallynon-absorbing of wavelengths from 0.3 to 2.5 μm. In someimplementations, the hydrophobic material includes fluorinated silicananospheres. In some implementations, the hydrophobic material includesnano-etched silica.

In one implementation, the coating 120 has a thickness of at least 70μm.

In certain implementations, the coating 120 provides passive radiativecooling of 50 to 100 W/m² when cooling a surface at 30 degrees C. at anambient air temperature of 30 degrees C., without input of fluids suchas water, air, or refrigerant, or electrical or mechanical energy. Incertain implementations, the coating provides passive radiative coolingof up to about 10° C. below ambient when solar illumination is presentand without input of water or electricity (e.g., at noon in September inPalo Alto, Calif., for an ambient temperature of about 30° C.). Incertain implementations, a tent with coating 120 on an upper surface ofthe tent is up to about 11° C. cooler than an uncoated tent (e.g., inAugust and September) (e.g., tent surface temperature, temperatureinside the tent, etc.). The coating 120 provides the passive radiativecooling, when the coating is exposed to the sky at normal incidence andat an angle. The coating 120 provides the passive radiative cooling whennot exposed to the sun and when exposed to the sun.

FIG. 1A illustrates an apparatus 100 a including a coating 120 of aradiative cooling formulation on a substrate 110, in accordance withimplementations of the present disclosure.

FIGS. 1B-D illustrate an apparatus 100 including one or more layers onthe coating 120. In certain implementations, the one or more layersprovide one or more of resistance to abrasion, ultraviolet radiation,water, or inorganic pollutants. The coating 120 with or without the oneor more layers spontaneously cools down below ambient temperature whenexposed to the sky, even when solar illumination is present.

FIG. 1B illustrates an apparatus 100 b including a coating 120 of aradiative cooling formulation on a substrate 110 and a first layer 130on the coating, in accordance with implementations of the presentdisclosure. In some implementations, the first layer 130 comprises oneor more of a hydrophobic material, TiO₂, or a fluoropolymer (e.g., PTFE,PFA, FEP, ETFE, THV, etc.). The fluoropolymer may be substantiallynon-absorbing of wavelengths from 0.3 to 2.5 μm (e.g., the fluoropolymermay have measureable, yet insignificant absorption in part of thiswavelength range). In some implementations, a layer of fluoropolymerincludes ETFE and the ETFE may be a Tefzel™ cover sheet that protectsthe coating 120 from environmental degradation. ETFE is hydrophobic andis transmissive to solar radiation (e.g., ETFE decreases solarreflectance by about 0.28%, ETFE is substantially non-absorbing ofsunlight). In some implementations, the layer 130 includes componentsfor resistance to one or more of abrasion (e.g., an anti-abrasion layerthat protects coating 120 from degradation by sand or other particles),UV radiation, water, dust, or dirt.

FIG. 1C illustrates an apparatus 100 c including a coating 120 of aradiative cooling formulation on a substrate 110, a first layer 130 onthe coating 120, and a second layer 140 on the first layer 130, inaccordance with implementations of the present disclosure. In someimplementations, the first layer 130 is one or more of a hydrophobicmaterial or TiO₂. In some implementations, the second layer 140 is anhydrophobic material, TiO₂, or a fluoropolymer (e.g., a Tefzel™ coversheet).

FIG. 1D illustrates an apparatus 100 d including a coating 120 of aradiative cooling formulation on a substrate 110, a first layer 130 onthe coating 120, a second layer 140 on the first layer 130, and a thirdlayer 150 on the second layer 140, in accordance with implementations ofthe present disclosure. In sonic implementations, the first layer 130 isan hydrophobic material and the second layer 140 is TiO₂. In someimplementations, the first layer is TiO₂ and the second layer 140 is anhydrophobic material. In some implementations, the third layer 150 is afluoropolymer (e.g., a Tefzel™ cover sheet). One or more of the firstlayer 130, the second layer 140, or the third layer 150 may besubstantially non-absorbing of wavelengths from 0.3 to 2.5 μm (e.g., theone or more layers may have measureable, yet insignificant absorption inpart of this wavelength range).

FIG. 2A is a graph 200 illustrating power absorbed 202 by an apparatus100 with a coating 120 of a radiative cooling formulation due toincident atmospheric thermal radiation, power radiated 206 (P_(rad)) bythe apparatus 100, solar power 204 (P_(sun)) incident on the apparatus100, and net cooling power 208 of the apparatus 100 at surfacetemperatures, in accordance with implementations of the presentdisclosure.

The net cooling power 208 (P_(cooling)) of the apparatus 100 with acoating 120 of a radiative cooling formulation may be calculated usingequations (1)-(6).

P _(cooling) =P _(rad) −P _(atm) −P _(sun) −P _(con)   (1)

P_(cooling) is the net cooling power 208. P_(rad) is the power radiated206 out by the apparatus 100. P_(atm) is the power absorbed 202 due toincident atmospheric thermal radiation (e.g., at ambient air temperature222). P_(sun) is the solar power 204 incident on apparatus 100. P_(con)is the power lost or gained due to convection and conduction. Powerradiated 206 may be calculated using equation (2).

P _(rad)(T)=∫d Ω cos θ∫ ₀ ^(∞) dλI _(BB)(T, λ)ε(λ, θ)   (2)

T is the temperature of the apparatus 100. The integral of dΩ is theangular integral over a hemisphere. The integral of I_(BB)(λ,θ) is theintegral of the spectral radiance of a blackbody at temperature T.ε(λ,θ) is the spectral and angular emissivity.

Power absorbed 202 may be calculated using equation (3).

P _(atm)(T _(atm))=∫dΩ cos θ∫₀ ^(∞) dλI _(BB)(T _(atm), λ)ε(λ,θ)ε_(atm)(λ, θ)   (3)

T_(atm) is the atmospheric temperature. ε_(atm)(λ,θ) is spectral andangular emissivity due to incident atmospheric thermal radiation and maybe calculated using equation (4).

ε_(atm)(λ,θ)=1−t(λ)^(1/cos θ)   (4)

t(λ) is the atmospheric transmittance in the zenith direction.

Solar power 204 may be calculated using equation (5).

P _(sun)=∫₀ ^(∞) dλI _(AM1.5)(λ)ε(λ, θ)   (5)

P_(sun) is the incident solar power absorbed by the apparatus 100.I_(AM1.5) is the solar illumination in the AM1.5 spectrum. ε(λ,θ) isemissivity of the apparatus 100 at a fixed angle (e.g., angle theapparatus 100 is facing the sun).

Power lost or gained due to convection and conduction may be calculatedusing equation (6).

P _(con)(T, T _(amb))=h _(c)(T _(amb) −T)   (6)

P_(con) is the power due to convection and conduction. h_(c) is acombined non radiative heat coefficient that captures the collectiveeffect of conductive and convective heating owing to the contact of theapparatus 100 with external surfaces (conduction) and air adjacent tothe apparatus 100 (convection).

Returning to FIG. 2A, in some implementations, the P_(cooling) 208 of anapparatus 100 with a coating 120 of a radiative cooling can becalculated by P_(rad) 206 minus P_(atm) 202 minus P_(sun) 204. Forexample. P_(cooling) 208 of apparatus 100 with a surface temperature of8° C. is about −100 W/m², at 20° C. is about 0 W/m², at 26° C. is about100 W/m², at 30° C. is about 123 W/m², at 40° C. is about 240 W/m², at50° C. is about 350 W/m², at 54° C. is about 400 W/m². P_(cooling) 208may be at least 100 W/m² when the surface temperature of the apparatus100 is at or above 28° C.

FIG. 2B is a graph 220 illustrating ambient temperature 222 andtemperature of an apparatus 100 including a coating 120 of a radiativecooling formulation on a substrate 110 at different times of day, inaccordance with implementations of the present disclosure. Thetemperature of the apparatus 100 is about 8-9° C. lower than ambienttemperature 222.

FIGS. 3A-E are graphs illustrating linear regression of change oftemperature of cool roof paint 310 and linear regression of change oftemperature of a coating 120 of a radiative cooling formulation atdifferent parameters, in accordance with implementations of the presentdisclosure. The performance of the coating 120 may depend on severalparameters that are correlated. The temperature of the coating 120 islower than the temperature of the cool roof paint for a variety ofdifferent parameters as shown in FIGS. 3A-E.

FIG. 3A is a graph 300 illustrating linear regression of change oftemperature of cool roof paint 310 and linear regression of change oftemperature of a coating 120 of a radiative cooling formulation atdifferent values of solar illumination, in accordance withimplementations of the present disclosure. The coating 120 has a linearregression of about 3.5° C. degrees below ambient temperature 222 atabout 20 W/m² to about 3.3° C. degrees below ambient temperature 222 atabout 800 W/m². The cool roof paint 310 has a linear regression of about3.3° C. degrees below ambient temperature 222 at about 20 W/m² to about1.2° C. degrees above ambient temperature 222 at about 800 W/m². Assolar illumination increases, the temperature difference between theambient temperature 222 and the temperature of the coating 120 stayssubstantially constant whereas as solar illumination increases, thetemperature of the cool roof paint 310 rises and becomes greater thanthe ambient temperature 222.

FIG. 38 is a graph 320 illustrating linear regression of change oftemperature of cool roof paint 310 and linear regression of change oftemperature of a coating 120 of a radiative cooling formulation atdifferent values of ambient temperature, in accordance withimplementations of the present disclosure. The coating 120 has a linearregression of about 2.9° C. degrees below an ambient temperature 222 ofabout 18° C. to about 4.4° C. degrees below an ambient temperature 222of about 36° C. The cool roof paint 310 has a linear regression of about0.3° C. degrees below an ambient temperature 222 of about 18° C. toabout 0.8° C. degrees above an ambient temperature 222 of about 36° C.As ambient temperature 222 increases, the temperature of the coatingdecreases whereas as ambient temperature 222 increases, the temperatureof the cool roof paint 310 rises and becomes greater than the ambienttemperature 222.

FIG. 3C is a graph 340 illustrating linear regression of change oftemperature of cool roof paint 310 and linear regression of change oftemperature of a coating 120 of a radiative cooling formulation atdifferent values of relative humidity, in accordance withimplementations of the present disclosure. The coating 120 has a linearregression of about 4.0° C. degrees below ambient temperature 222 atabout 2% relative humidity to about 2.5° C. degrees below ambienttemperature 222 at about 100% relative humidity. The cool roof paint 310has a linear regression of about 0.8° C. degrees above ambienttemperature 222 at about 2% relative humidity to about 1.4° C. degreesbelow ambient temperature 222 at about 100% relative humidity. Thetemperature of the coating 120 is lower (e.g., 1-5° C. lower) than thetemperature of the cool roof paint 310 for all values of relativehumidity shown in FIG. 3C.

FIG. 3D is a graph. 360 illustrating linear regression of change oftemperature of cool roof paint 310 and linear regression of change oftemperature of a coating 120 of a radiative cooling formulation atdifferent values of wind speed, in accordance with implementations ofthe present disclosure. The coating 120 has a linear regression of about4.0° C. degrees below ambient temperature 222 at about 0.2 meters/second(m/s) wind speed to about 2.3° C. degrees below ambient temperature 222at about 2.1 m/s wind speed. The cool roof paint 310 has a linearregression of about 0.9° C. degrees below ambient temperature 222 atabout 0.2 m/s wind speed to about 1.6° C. degrees above ambienttemperature 222 at about 2.1 m/s wind speed. The temperature of thecoating 120 is lower (e.g., about 3.1-3.9° C. lower) than thetemperature of the cool roof paint 310 for 0.2 to 2.1 m/s wind speed.

FIG. 3E is a graph 380 illustrating linear regression of change oftemperature of cool roof paint 310 and linear regression of change oftemperature of a coating 120 of a radiative cooling formulation atdifferent values of cloud cover, in accordance with implementations ofthe present disclosure. The coating 120 has a linear regression of about3.7° C. degrees below ambient temperature 222 at about 0 Oktas e.g,completely clear sky per a weather station of the National Oceanic andAtmospheric Administration (NOAA)) to about 2.9° C. degrees belowambient temperature 222 at about 8 Oktas (e.g., the sky is completelycovered in clouds per the weather station of the NOAA). The cool roofpaint 310 has a linear regression of about 0.4° C. degrees above ambienttemperature 222 at about 0 Oktas to about 1.3° C. degrees below ambienttemperature 222 at about 8 Oktas. The temperature of the coating 120 islower (e.g, about 1.6-4.1° C. lower) than the temperature of the coolroof paint 310 for 0-8 Oktas.

FIG. 4A is a graph 400 illustrating emissivity at different wavelengthsfor poly ethyl methacrylate (PEMA) and polyvinyl butyral (PVB), inaccordance with implementations of the present disclosure. The radiativecooling formulation of coating 120 includes a binder that includes twoor more polymers. In some implementations, the binder includes a firstpolymer that is PEMA and a second polymer that is PVB. PEMA and PVB arepractically water insoluble and are substantially non-absorbing to lighthaving wavelengths in the solar spectrum. PEMA and PVB have emissivitypeak values greater than 0.85 at wavelengths between 4 and 35 μm. Forexample, PEMA has emissivity peak values of about 0.98 at about 5.8 μm,about 0.88 at about 6.8 μm, about 0.89 at about 6.9 μm, about 0.98 atabout 7.9 μm, about 0.86 at about 8.4 μm, and about 0.88 at about 9.7μm. PVB has emissivity peak values of about 0.92 at about 3.4 μm, about0.90 at about 7.2 μm, and about 0.95 at about 9.8 μm. PEMA hasemissivity peak values (e.g., at wavelengths of about 5.8 μm, 6.8 μm,6.9 μm, 7.9 μm, 8.4 μm, and about 9.7 μm) that do not overlap with theemissivity peak values of PVB (e.g., at wavelengths of about 3.4 μm, 7.2μm, and 9.8 μm). A net emissivity a coating 120 including a binder thatincludes PFMA and PVB is greater than a coating including a binder thatincludes only PEMA or PVB as shown in FIG. 4B.

FIG. 48 is a graph 420 illustrating temperature of a coating 422including one polymer, temperature of a coating 120 of a radiativecooling formulation including two polymers, and ambient temperature atdifferent times of day, in accordance with implementations of thepresent disclosure. In one implementation, a first object and a secondobject are aluminum plates (e.g., aluminum plates with substantiallysimilar properties and dimensions). The coating 422 of the first objectand the coating 120 of the second object may include BaSO₄. The firstobject is coated with a first radiative cooling formulation (e.g.,passive radiative cooling paint) that contains a binder that includesonly one polymer (e.g., only a PVB polymer, only a PVB binder) and thesecond object is coated with a second radiative cooling formulation(e.g., passive radiative cooling paint, coating 120) that includes bothPVB and PEMA polymers in the binder (e.g., paint with a binder mixtureof two or more polymers). The second object which is coated with amulti-polymer binder is about 0.1-8.4° C. cooler than the first objectwhich is coated with a single-polymer binder. The sample temperaturesshown in graph 300 may be for a summer day when the sun is at thezenith.

FIG. 4C is a graph 440 illustrating solar reflectance of coatings 120 ofa radiative cooling formulation of different thicknesses, in accordancewith implementations of the present disclosure. In one implementation,the coating 120 has a thickness of at least 70 μm. In anotherimplementation, the coating 120 in FIG. 4C has a thickness of about 70μm. In some implementations, the coating 120 may have a solarreflectance of about 0.8 at a thickness of 6 μm, 0.84 at 8 μm, 0.9 at 26μm, 0.94 at 56 μm, 0.88 from about 127 μm to about 145 μm, 0.89 fromabout 149 μm to about 195 μm, 0.91 from about 240 μm to about 250 μm,and about 0.915 at about 270 μm. In some implementations, the coating120 has a solar reflectance of greater than 0.95 at thicknesses of 70 μmor greater. In some implementations, the BaSO₄ particles has aD50(volume) of 2 μm.

FIG. 4D is a graph 460 illustrating solar reflectance of coatings 120 ofa radiative cooling formulation including solar reflector material ofdifferent particle sizes, in accordance with implementations of thepresent disclosure. The coating 120 includes a solar reflector material.In some implementations, the solar reflector material includes particlesof BaSO₄. In some implementations, at least half of the particles ofBaSO₄ in coating 120 are smaller than 2 μm. In some implementations, atleast 75% of the particles of BaSO₄ in coating 120 are smaller than 2μm. In some implementations, at least 90% of the particles of BaSO₄ incoating 120 are smaller than 2 μm. The particles may have dimensionssmaller or equal to two times the wavelengths to be reflected in thesolar radiation range to maximize the solar reflectance of the coating120. The solar reflectance is about 0.970 to 0.982 at a peak particlesize of about 0.2 μm, about 0.967 to 0.978 at a peak particle size ofabout 1.4 μm, about 0.964 to 0.975 at a peak particle size of about 1.8μm, and about 0.951 to 0.961 at a peak particle size of about 4.4 μm. Insome implementations, the particles are 0.2 μm or larger. In someimplementations, the particles are 0.1 or larger. In someimplementations, the particles are no larger than 5 μm.

FIG. 5A is a flow diagram illustrating a method 500 of applying acoating 120 of a radiative cooling formulation to an object, inaccordance with implementations of the present disclosure.

At block 510, a coating 120 of a radiative cooling formulation isapplied to an object. In certain implementations, the object comprisesat least one of aluminum, steel, carbon fiber resin, roof tent, flexibletarp, or surface of an automobile. In one implementation, the coating120 is brushed on the object. In another implementation, the coating 120is sprayed on the object. In another implementation, the coating 120 isrolled on the object. In another implementation, the coating 120 isapplied to the object by dipping (e.g., dipping at least a portion ofthe object in a radiative cooling formulation). In anotherimplementation, the coating 120 is doctor bladed (e.g., knife coated,blade coated) on the object (e.g., radiative cooling formulation isplaced on a surface of the object and at least one of a blade or theobject move relative to each other so that the blade controls thethickness of the coating 120 on the object via a gap size between theblade and the surface of the object). In another implementation, thecoating, is applied to the object by another painting technology. Theradiative cooling formulation includes a binder and a solar reflectormaterial embedded in the binder. The binder includes at least twopolymers that are practically water insoluble and are substantiallynon-absorbing to light having wavelengths in a solar spectrum.

FIG. 58 is a flow diagram illustrating a method 540 of applying acoating 120 of a radiative cooling formulation to an object and one ormore layers (e.g., first layer 130, second layer 140, third layer 150,etc.) on the coating 120, in accordance with implementations of thepresent disclosure.

At block 550, a primer (e.g., primer layer) is applied to an object. Inone implementation, the primer is for adhesion (e.g., improves adhesionbetween the object and the coating 120). In another implementation, theprimer is for corrosion resistance (e.g., prevents corrosion of theobject (e.g., a metal object)).

At block 560, a coating 120 of a radiative cooling formulation isapplied to the primer (e.g., on the primer, on the object). Block 560may be similar to block 510 of FIG. 5A.

At block 570, a first layer 130 including TiO₂ is applied to the coating120 (e.g., on the primer, on the object, on the coating 120). The firstlayer 130 may be applied by one or more of brushing, spraying, rolling,dipping, doctor blading a paint, or another painting technology.

At block 580, a second layer 140 including a hydrophobic material isapplied to the first layer 130 (e.g., on the primer, on the object, onthe coating 120, on the first layer 130). In one implementation, thefirst layer 130 includes a hydrophobic material and the second layer 140includes TiO₂. The second layer 140 may be applied by one or more ofbrushing, spraying, rolling, dipping, doctor blading a paint, or anotherpainting technology.

At block 590, a third layer 150 including fluoropolymer (e.g.. PTFE,PFA, FEP, ETFE, THV, etc.) is applied to the second layer 140 (e.g., onthe primer, on the object, on the coating 120, on the first layer 130,on the second layer 140). The third layer 150 may be applied by one ormore of brushing, spraying, rolling, dipping, doctor blading a paint, oranother painting technology. In certain implementations, the third layeris applied by laminating the third layer on the second layer 140.

In one implementation, one or more of the first layer 130, second layer140, and third layer 150 are mixed together. In another implementation,one or more of the first layer 130, second layer 140, and third layer150 are mixed in with the coating 120 of a radiative coolingformulation. In another implementation, one or more of the first layer130, second layer 140, and third layer 150 is omitted (e.g., the secondlayer 140 is on the coating 120, the third layer 150 is on the coating120, etc.). In another implementation, one or more of the coating 120,first layer 130, second layer 140, and third layer 150 is on the object(e.g., substrate 110) in a different order that the order illustrated inFIG. 5B.

For simplicity of explanation, the methods of this disclosure aredepicted and described as a series of acts. However, acts in accordancewith this disclosure can occur in various orders and/or concurrently,and with other acts not presented and described herein. Furthermore, notall illustrated acts may be required to implement the methods inaccordance with the disclosed subject matter.

Although implementations of the disclosure were discussed in the contextof applying a coating 120 to an object, one or more of the components ormaterials described herein may be utilized in other passive radiationcooling systems. Thus, implementations of the disclosure are not limitedto a coating on an object.

In the foregoing description, numerous details were set forth. It willbe apparent, however, to one of ordinary skill in the art having thebenefit of this disclosure, that the implementations of the presentdisclosure may be practiced without these specific details. In someinstances, certain structures and devices are shown in block diagramform, rather than in detail, in order to avoid obscuring the presentdisclosure. It is to be understood that the details of such structuresand devices, as well as various processes for producing the same, wouldbe within the purview of one of ordinary skill in the art.

The terms “above,” “under,” “between,” and “on” as used herein refer toa relative position of one layer with respect to other layers. As such,for example, one layer deposited or disposed above or under anotherlayer may be directly in contact with the other layer or may have one ormore intervening layers. Moreover, one layer deposited or disposedbetween layers may be directly in contact with the layers or may haveone or more intervening layers. In contrast, a first layer “on” ordeposited “onto” a second layer is in contact with that second layer.Additionally, the relative position of one layer with respect to otherlayers is provided assuming the initial disk is a starting substrate andthe subsequent processing deposits, modifies and removes films from thesubstrate without consideration of the absolute orientation of thesubstrate. Thus, a film that is deposited on both sides of a substrateis “over” both sides of the substrate.

The words “example” or “exemplary” are used herein to mean serving as anexample, instance, or illustration. Any aspect or design describedherein as “example” or “exemplary” is not necessarily to be construed aspreferred or advantageous over other aspects or designs. Rather, use ofthe words “example” or “exemplary” is intended to present concepts in aconcrete fashion. As used in this application, the term “or” is intendedto mean an inclusive “or” rather than an exclusive “or”. That is, unlessspecified otherwise, or clear from context, “X includes A or B” isintended to mean any of the natural inclusive permutations. That is, ifX includes A; X includes B; or X includes both. A and B, then “Xincludes A or B” is satisfied under any of the foregoing instances. Inaddition, the articles “a” and “an” as used in this application and theappended claims should generally be construed to mean “one or more”unless specified otherwise or clear from context to be directed to asingular form. Reference throughout this specification to “animplementation” or “one implementation” means that a particular feature,structure, or characteristic described in connection with theimplementation is included in at least one implementation. Thus, theappearances of the phrase “an implementation” or “one implementation” invarious places throughout this specification are not necessarily allreferring to the same implementation.

The present disclosure is not to be limited in scope by the specificimplementations described herein. Indeed, other various implementationsof and modifications to the present disclosure pertaining to laserdevices, in addition to those described herein, will be apparent tothose of ordinary skill in the art from the preceding description andaccompanying drawings. Thus, such other implementations andmodifications pertaining to laser devices are intended to tall withinthe scope of the present disclosure. Further, although the presentdisclosure has been described herein in the context of a particularimplementation in a particular environment for a particular purpose,those of ordinary skill in the art will recognize that its usefulness isnot limited thereto and that the present disclosure may be beneficiallyimplemented in any number of environments for any number of purposes.Accordingly, the claims set forth below should be construed in view ofthe full breadth and spirit of the present disclosure as describedherein, along with the full scope of equivalents to which such claimsare entitled.

It will be appreciated that variants of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be combined intomany other different systems or applications. Various presentlyunforeseen or unanticipated alternatives, modifications, variations, orimprovements therein may be subsequently made by those skilled in theart which are also intended to be encompassed by the following claims.

What is claimed is:
 1. An apparatus comprising: a radiative coolingformulation comprising: a first component with >55% reflectance in awavelengths range of 0.3 to 2.5 microns; a second component with >0.85peak thermal emissivity in a window range of 4 to 35 microns; and athird component to mechanically bind together a mixture of the first andsecond components.
 2. The apparatus of claim 1, wherein the radiativecooling formulation further comprises: a fourth component to protect thefirst and second components from at least one of physical damage ordegraded properties.
 3. The apparatus of claim 1, wherein two or more ofthe first, second and third components comprise a same material.
 4. Theapparatus of claim 2, wherein the second component and the fourthcomponent are the same component.
 5. The apparatus of claim 1, whereinthe first component comprises one or more of the following materials:Teflon PTFE, barium sulfate, zinc oxides, aluminum oxides, magnesiumoxides, TiO₂.
 6. The apparatus of claim 1, wherein the second componentcomprises one or more of the following materials: materials withcarbon-carbon bonding, PTFE, PFA, FEP, ETFE, THV, Tefzel™, ethylcellulose, poly ethyl methacrylate PEMA, poly methyl methacrylate PMMA,polyvincyl butyrol PVB, cellulose acetate, polyethylene, polypropylene,polyethylene terephthalate PET, polyethylene naphthalate PEN,polyesters, and polycarbonate.
 7. The apparatus of claim 1, wherein thesecond component is a top coating.
 8. The apparatus of claim 1, whereinthe second component comprises one or more of the following materials:PTFE, teflon, PFA, FEP, ETFE, THV, Tefzel™, ethyl cellulose, poly ethylmethacrylate PEMA, poly methyl methacrylate PMMA, polyvincyl butyrolPVB, cellulose acetate, polyethylene, polypropylene, polyethyleneterephthalate PET, polyethylene naphthalate PEN, polyesters, andpolycarbonates.
 9. The apparatus of claim 2, wherein the fourthcomponent comprises one or more of the following materials: PTFE,teflon, PFA, FEP, ETFE, THV, Tefzel™, ethyl cellulose, poly ethylmethacrylate PEMA, poly methyl methacrylate PMMA, polyvincyl butyrolPVB, cellulose acetate, polyethylene, polypropylene, polyethyleneterephthalate PET, polyethylene naphthalate PEN, polyesters, andpolycarbonates.
 10. The apparatus of claim 1, wherein radiative coolingformulation is comprises of a single layer having the first, second andthird components mixed therein.
 11. The apparatus of claim 1, whereinradiative cooling formulation is comprises of a multilayer structurehaving the first, second and third components mixed therein.
 12. Theapparatus of claim 1, wherein the first component has >95% reflectancein the wavelengths range of 0.3 to 2.5 microns.
 13. The apparatus ofclaim 1, wherein the radiative cooling formulation forms a coating thatprovides passive radiative cooling greater than zero Watts per squaremeter (W/m²) without input of water or electricity.
 14. The apparatus ofclaim 1, wherein the radiative cooling formulation forms a coating thatprovides a passive radiative cooling of at least 5 degrees Celsius belowambient temperature when solar illumination is present on the apparatusand without input of water or electricity.
 15. The apparatus of claim 2,wherein the fourth component is physically robust but does not decreaseradiative flux.
 16. The apparatus of claim 2, wherein at least one ofthe second component and fourth component is a clear coating that isradiatively emissive in a window range of 4-35 microns, but issubstantially non-absorbing of visible wavelengths, such that theoptical properties of the surface are largely unchanged by the additionof the clear coat, but the thermal emissivity is enhanced.